1. Field of the Invention
The present invention relates to an optical transmission system, and in particular to a wavelength division multiplexing (WDM) optical transmission system.
In the wavelength multiplexing optical transmission system, not only an optical transmission between a transmission end and a reception end, but also an OADM (Optical Add/Drop Multiplexer) function of adding/dropping (inserting/branching) a part of a signal kept in an optical state at intermediate nodes is required for structuring a flexible optical network at low cost.
2. Description of the Related Art
FIG. 12A shows a prior art example of such an optical network. Namely, relay nodes (Back-to-Back terminals having add/drop function) N#1 and N#2 are connected between a transmitter Tx and a receiver Rx. An optical signal is relayed and amplified in a transmission line between the transmitter Tx and the relay node N#1, the relay nodes N#1 and N#2, and the relay node N#2 and the receiver Rx by passing through booster amplifiers BA#1-BA#3, linear optical amplifying relays ILA (InLine Amplifiers) #1-#14, and reception amplifiers RA#1-RA#3 as shown in FIG. 12A.
At a transmission end, a wave multiplexer (wave multiplexer/demultiplexer) CW1 multiplexes channel signals having different wavelengths from the transmitter Tx, thereby performing wavelength multiplexing. At a reception end, a wave demultiplexer (wave multiplexer/demultiplexer) CW2 demultiplexes channel signals from the wavelength-multiplexed optical signal to be provided to the receiver Rx.
Also, between the transmitter Tx and the receiver Rx, optical signals of some channels are dropped/added as they are, or passed through as they are at the relay nodes N#1 and N#2.
Thus, in a wavelength multiplexing optical transmission system, optical parts such as wave multiplexers/demultiplexers and optical amplifiers are connected between the transmitter Tx and the receiver Rx. Since a level difference (tilt) between the channel signals influences a transmission quality, the Japanese Patent Application Laid-open No. 10-276173 has proposed a method by which the wave-multiplexed optical signal is monitored, an attenuation of a variable optical attenuator per channel arranged before wave multiplexing is adjusted based on a result of a level measurement by a tunable filter, and inter-level deviations of the channel signals are adjusted.
[Patent Document 1]
Japanese Patent Application Laid-Open No. 10-276173
(Abstract, FIG. 1)
On the other hand, when a pass characteristic from the transmission end where the transmitter Tx is arranged to the reception end where the receiver Rx is arranged is measured in the optical network shown in FIG. 12A, with attention to a certain wavelength λn within the wavelength-multiplexed optical signals, as shown in FIG. 12B, a transmission line pass loss assumes the minimum at the point deviated toward e.g. the right side from a wavelength λn within a modulation band of the optical signal wavelength. Therefore, the transmission line pass characteristic (tangent line) F tilts at a center wavelength λn and is not flat, thereby deteriorating the transmission quality.
This will now be further described referring to FIGS. 13A-13E. If the pass characteristic is flat from the transmission end to the reception end as mentioned above, a symmetric modulation spectrum with respect to the center wavelength λn is given, as shown in FIG. 13A. However, when a non-flat pass characteristic having a fixed gradient angle as shown in FIGS. 12B and 13B occurs, a modulation spectrum is not symmetric but distorted as shown in FIG. 13C.
As a result, when the modulation spectrum is normal as shown in FIG. 13A, a wide distortionless eye pattern can be obtained as shown in FIG. 13D. However, when the modulation spectrum is distorted as shown in FIG. 13C, the eye pattern is also distorted as shown in FIG. 13E, thereby causing a signal error.
As for such non-flatness of the transmission line pass characteristic within the modulation band, it is required especially in the wave multiplexer/demultiplexer that the pass characteristic of the signal wavelength band is flat, channel signals of other wavelengths are fully suppressed, and isolation shown in FIG. 12B is secured. However, since the wavelength multiplexing optical transmission system with a narrow wavelength interval and a high density is adopted in order to increase a transmission capacity as mentioned above, both become hardly compatible together with the wavelength interval narrowed. Also, by interposition of the add/drop node, the number of optical parts through which the optical signals pass increases, and the non-flat pass characteristic within the modulation band is weighted and becomes serious.
Such non-flatness of the pass characteristic depends on individual characteristics of parts and relative relationships between wavelengths of the optical signals and the pass characteristic of the parts, giving no specific characteristic, so that such a problem has been usually addressed by making part specifications strict, limiting the number of parts passed therethrough, or allowing a transmission quality deterioration (penalty).